The moisture sensitivity of corrugated board structural adhesives continues to be a difficult problem for those using shipping or storage containers made of corrugated paperboard. Corrugated board is made by gluing paper members with a starch-based adhesive composition. The resulting corrugated board product can lose strength due to the effect of humidity, mist, rain or standing water on both the starch adhesive and the paper members which can be hydrophilic or water sensitive.
Starch-based adhesives are used in paperboard manufacture by coating an aqueous starch suspension onto corrugated paperboard members, contacting the members and heating the suspension to cure the adhesive bond. The starch molecules bond the paper members through hydrogen bonds between the starch polysaccharides and the polysaccharides in the paper members. However, the adhesive can remain somewhat hydrophilic and can absorb substantial amounts of water. The starch in the presence of such water can return to a suspension or emulsion of starch in the water derived from the environment and as a result can rapidly lose bond strength. Similarly, the paper used in making corrugated board is generally formed from a water slurry of cellulose fibers derived from wood or other plant sources which is made into a sheet by removing water from the sheeted slurry. The sheet is commonly rolled and dried to remove residual water resulting in finished paper. The resulting paper can be water sensitive since its component parts are hydrophilic and can tend to revert partly, in the presence of water, to the dispersion of cellulose particles in water from which it was prepared.
Many adhesives for preparing corrugated board have been proposed in the past including aqueous starch-based adhesives, hot melt thermoplastic adhesives, hot melt coated string-like adhesives, and adhesives comprising aqueous or other emulsions of synthetic polymers.
The preferred adhesive, in commercial use today, is a starch based adhesive with moisture or water resistance derived from formulations including resorcinol-formaldehyde resins, phenol-formaldehyde resins, calcium cyanamide-formaldehyde resins, urea-formaldehyde resins, and ketone-formaldehyde resins. Such resins are chemically reactive and tend to increase water resistance of the bond line by reacting with and crosslinking starch molecules through hydroxyl groups. By crosslinking the starch molecules, hydrophobicity of the bond line is increased since the molecular weight of the starch is increased through crosslinking and the number of available hydrophilic hydroxyl groups are reduced through the crosslinking reaction. Accordingly, the chemical reactivity of the water resistant resins is important in imparting water resistance. However the reactivity of the resins can cause problems in both adhesive formulation and use.
In the manufacture of resin containing water resistant starch-based corrugating non-borated carrier corrugating adhesives, during and after the addition of the water resistant resin composition to starch slurries, the viscosity of the material can show a significant increase which can last for fifteen to sixty minutes after preparation. In borated carrier corrugating adhesives the viscosity increase can last for much longer times and can be permanent. Note that borax is present in all such adhesives. The primary difference is in the location of the boron compounds. Depending on preparation methods, the boron compounds can end in either the carrier phase or the secondary phase. Such increase can be a significant detriment in the mixing, pumping and application of the starch-based adhesives during adhesive preparation and corrugated board manufacture in corrugating machines. Such unwanted rapid viscosity increase is known as "resin-shock" in the corrugated paperboard industry. While we do not wish to be held to a theory of any aspect of this invention, persons skilled in this art believe that resin-shock is a result of an interaction between the resin (including resin impurities and by-products) and the starch in the adhesive composition.
Accordingly, a substantial need exists in the art to prevent "resin-shock" during the manufacture of starch-based adhesives with water-resistant resins, without having a significant negative impact on the manufacturing aspects or ease of use of the adhesive compositions or the quality of the resulting corrugated board. More particularly, it is important that the machining, viscosity, and bond strength of the adhesive in wet and dry corrugated paperboard be maintained in the absence of significant "resin-shock".